1. Field of the Invention
The present invention relates to a fuel cell stack comprising a plurality of fuel cell units each composed of an electrolyte interposed between an anode electrode and a cathode electrode, the plurality of fuel cell units being stacked with a plurality of separators intervening therebetween.
2. Description of the Related Art
For example, the solid polymer type fuel cell comprises a fuel cell unit including an anode electrode and a cathode electrode disposed opposingly on both sides of an electrolyte composed of a polymer ion exchange membrane (cation exchange membrane), the fuel cell unit being interposed between separators. Usually, the solid polymer type fuel cell is used as a fuel cell stack comprising a predetermined number of the fuel cell units and a predetermined number of the separators which are stacked with each other.
In such a fuel cell, a fuel gas such as a gas principally containing hydrogen (hereinafter referred to as xe2x80x9chydrogen-containing gasxe2x80x9d), which is supplied to the anode electrode, is converted into hydrogen ion on the catalyst electrode, and the ion is moved toward the cathode electrode via the electrolyte which is appropriately humidified. The electron, which is generated during this process, is extracted for an external circuit, and the electron is utilized as DC electric energy. An oxygen-containing gas such as a gas principally containing oxygen (hereinafter referred to as xe2x80x9coxygen-containing gasxe2x80x9d) or air is supplied to the cathode electrode. Therefore, the hydrogen ion, the electron, and the oxygen gas are reacted with each other on the cathode electrode, and thus water is produced.
In order to supply the fuel gas, the oxygen-containing gas, and the cooling medium to the respective fuel cell units, the fuel cell stack usually adopts an internal manifold which is incorporated into the inside of the fuel cell stack, or an external manifold which is externally attached to the fuel cell stack. Such a manifold is arranged corresponding to the upper and lower sides or the right and left sides of the respective fuel cell units. When the fluid such as the fuel gas, the oxygen-containing gas, or the cooling medium is supplied to a supply passage from one end side in the stacking direction of the stack, then the fluid is supplied to each of the fuel cell units via respective branch passages, and then the fluid is merged into a discharge passage to be discharged to the one end side or the other end side in the stacking direction of the stack.
Specifically, as shown in FIG. 12, the constitutive elements of a fuel cell-stack 1 are classified into fuel cell units 2 and a flow passage 3 of the counter flow type for delivering and supplying the fluid to the respective fuel cell units 2. The flow passage 3 comprises a delivery tube 4 provided integrally with the manifold, branch tubes 5 for supplying the fluid in the direction of the arrow B perpendicular to the flow direction (direction of the arrow A) of the delivery tube 4, and a collective tube 6 for discharging the fluid merged from the respective branch tubes 5 in the direction of the arrow C which is opposite to the direction of the arrow A.
On the other hand, a fuel cell stack la shown in FIG. 13 is classified into fuel cell units 2a and a flow passage 3a of the parallel flow type. The flow passage 3a comprises a delivery tube 4a for supplying the fluid in the direction of the arrow A, branch tubes 5a for supplying the fluid along the respective fuel cell units 2a, and a collective tube 6a for discharging the fluid merged from the respective branch tubes 5a in the direction of the arrow A.
However, in the case of the flow passage 3 of the counter flow type shown in FIG. 12, the boundary portion between the delivery tube 4 and the branch tube 5 constitutes a communication passage which is formed to be at the right angle. As a result, the branch loss of the fluid pressure is large. Further, each of the branch tubes 5 communicates with the collective tube 6 via a communication passage which is formed to be at the right angle. Therefore, the merging loss of the fluid pressure is also large. FIG. 14 shows the difference between the inlet pressure of the delivery tube 4 and the outlet pressure of the collective tube 6, i.e., the difference in pressure concerning the branch tube 5. Therefore, such a problem is pointed out that the pressure loss in the flow passage 3 is considerably increased, and the fluid is unevenly supplied to the respective fuel cell units 2.
On the other hand, in the case of the flow passage 3a of the parallel flow type shown in FIG. 13, the outlet side pressure of the collective tube 6a is considerably lowered as compared with the inlet side pressure of the delivery tube 4a. Therefore, as shown in FIG. 15, the pressure loss in the flow passage 3a is considerably increased.
In view of the above, for example, as disclosed in Japanese Laid-Open Patent Publication No. 8-213044, a fuel cell is known, which is provided with a fuel flow-adjusting member formed to have a predetermined thickness by a porous member for permeating the fuel, arranged at an inflow port provided with a gap, in a delivery passage having the inflow port for the fuel for delivering the fuel flowing from the inflow port to a plurality of cells. However, in the case of the conventional technique described above, the following problem is pointed out. That is, although the fuel flow-adjusting member adjusts the flow of the fuel in the delivery passage, the pressure loss of the entire system is increased due to the fuel flow-adjusting member, and the power generation efficiency is lowered.
A principal object of the present invention is to provide a fuel cell stack which makes it possible to effectively reduce the pressure loss in a flow passage, and deliver a fluid uniformly and smoothly to respective fuel cell units with a simple structure.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.